Synthetic Aperture Radar. Hugh Griffiths THALES/Royal Academy of Engineering Chair of RF Sensors University College London

Transcription

1 Synthetic Aperture Radar Hugh Griffiths THALES/Royal Academy of Engineering Chair of RF Sensors University College London CEOI Training Workshop Designing and Delivering and Instrument Concept 15 March 2010

2 Historical origins the idea of synthesising a large antenna from a succession of spatial samples has its origins in radioastronomy; but the idea of a synthetic aperture radar (i.e.an active system) was conceived by Carl Wiley in the USA in the 1950s, in the context of sideways-looking airborne radar. he recognised that the variation in Doppler shift of echoes from a target meant that the beamwidth of a sideways-looking radar could be artificially reduced by filtering (Doppler beam sharpening or unfocused aperture synthesis). the first spaceborne SAR was carried by NASA s SEASAT satellite in This only lasted for three months, when a power supply fault caused its early demise, but in that time it produced a wealth of data from land, ocean and ice surfaces. nowadays SAR is a powerful set of techniques, both for spaceborne remote sensing and for aircraft-borne highresolution surveillance. Current areas of interest include interferometry, spotlight mode, polarimetry, inverse SAR (ISAR),..

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4 SAR geometry radar r r 0 2 x 2r 0 r r 0 x x 2.2r 2 x r 0 2 target f D v d x r0 2d r0 2d x v d

5 Digital SAR processing For a medium resolution SAR system where the range compression and azimuth compression functions are independent, we may define the following SAR processor: Range compression FFT data samples in range multiply by range compression spectrum replica inverse FFT Transpose data to be azimuth contiguous (corner turn) Azimuth compression FFT data in azimuth multiply by azimuth compression spectrum replica inverse FFT Store or display image

6 SAR imaging of moving targets A moving target with a radial component of velocity v r results in an echo Doppler shift 2vf r 0 fd c The Doppler-shifted sequence of echoes is matched-filtered (with a small mismatch) with an azimuth shift x. r d fd 2vd r vr v f D x echo Doppler bandwidth = 2v/d synthetic aperture length = r /d

7 SAR imaging of moving targets (after Rufenach, C.L. et al., Interpretation of Synthetic Aperture Radar measurements of ocean currents, J. Geophys. Res., Vol.88 No.C3, February 1983).

8 Satellite-borne remote sensing radar SEASAT (1978) ERS-1 (1991), ERS-2 (1995) ENVISAT (2002)

9 ASTOR In December 1999, Raytheon Systems Ltd was awarded a contract for the development of the UK Ministry of Defence Airborne Stand- Off Radar (ASTOR). The system, which includes five Bombadier Global Express business jet aircraft fitted with a derivative of the Raytheon ASARS-2 radar, is an airborne battlefield or ground surveillance radar system which will be operational with the Royal Air Force and the British Army in the year ASTOR

10 Games we can play... Ultra high-resolution SAR (and target recognition) Change detection Coherent change detection Interferometry Polarimetry Low-frequency SAR (FOPEN)

11 Sandia National Laboratories Twin Otter (Ku-band) source: Sandia Corporation -http// 11

12 Ultra-high resolution image Example of 3-look image yielding 10 cm resolution Cantalloube, H. and Dubois-Fernandez, P. Airborne X-band SAR imaging with 10 cm resolution technical challenge and preliminary results, Proc. EUSAR 2004 Conference, pp

13 Spotlight mode maximum achievable resolution Take the expression for spotlight mode azimuth resolution : x 4sin 2 A practical maximum value for might be 30º, i.e. = 60º in which case x 2 In range, r c f0 2B 2B For a fractional bandwidth of 100% B f 0 1 so r 2 target

14 Ultra-high resolution image 14

15 Coherent Change Detection (CCD) To detect whether or not a change has occurred, two images are taken of the same scene, but at different times. These images are then geometrically registered so that the same target pixels in each image align. After the images are registered, they are cross correlated pixel by pixel. Where a change has not occurred between the imaging passes, the pixels remain correlated, whereas if a change has occurred, the pixels are uncorrelated. Of course, targets that are not fixed or rigid, such as trees blowing in the wind, will naturally decorrelate and show as having "changed." While this technique is useful for detecting change, it does not measure direction or the magnitude of change.

16 Interferometric SAR - geometry B interferometric phase 2.Q. r 2 r 1 r 2 (Q=1 for single-pass, 2 for double-pass) H r 1 2.Q. r B 2r Bcos r P z 2.Q. Bcos (valid at long range or for large) y

17 Interferometric SAR The Rapid Terrain Visualization (RTV) interferometric synthetic aperture radar sensor was designed and developed by Sandia National Laboratories for the Joint Precision Demonstration Project Office of the United States Army (Intelligence, Electronic Warfare & Sensors PEO). This radar provides the ability to generate highly accurate map products in real-time, including digital elevation models (DEMs), orthorectified SAR images, as well as a measure of the data quality.

18 SIR-C SAR images

19 Internal waves in SAR images

20 Internal waves in SAR images spaceborne radar illuminated zone surface / capillary waves thermocline internal wave tidal currents mean ocean surface bottom topography

21 CARABAS I Experimental Demonstrator

22 CARABAS II/LORA

23 Detection of Concealed Targets Aerial photo X-band SAR ESAR 3m res. VHF-band SAR CARABAS-II 3m res. Attenuation is significantly reduced in VHF-band SAR, but forest clutter is still a problem.

24 Conclusions SAR is a mature set of techniques, with numerous applications in geophysical remote sensing and in military surveillance As well as advances in hardware and techniques, the key thing is the ability to extract information from SAR imagery